The most general method for producing the above described dual phase structured steel sheets comprises reheating a thin steel sheet up to .gamma.+.alpha. zone by using a heat-treating apparatus of a prior continuous annealing line and then rapidly quenching the reheated steel sheet in the subsequent cooling step to transform .gamma. portion formed in the heating step into martensite (referred to as "reheating method" hereinafter). But it is essential for the reheating method to add one step for the heat treatment and is not advantageous in view of economy and productivity.
On the other hand, it has been known to directly obtain the dual phase structure in a hot-rolling step without effecting the separate heat treating step (referred to as "hot-rolling method" hereinafter) but the cold formability of the dual phase structured steel sheets produced in the prior hot-rolling method is far inferior to that of the steel sheets produced in the above described reheating method.
In order to improve the cold formability of the steel sheets having a high tensile strength of more than 50 kg/mm.sup.2, which are mainly used in automotive field hereafter, it is necessary that the yield ratio YR is less than 70%, preferably less than 65% and the following value M of the strength-elongation balance parameter which is clarified and proposed by the inventors as an indication of the cold formability EQU M=0.45TS+El
wherein TS is tensile strength (kg/mm.sup.2) and El is total elongation (%), is 60 or more, but the dual phase structured steel sheets produced in the prior hot-rolling method can not reach the level satisfying this value and these requirements are satisfied only by the above described reheating method.
In general, the yield ratio and the strength-elongation balance of the dual phase structured high tensile strength steel shets vary depending upon the mixing ratio of ferrite phase to the hard second phase, the state of the hard second phase dispersed and ferrite grain size and the like and in order to obtain the above described yield ratio and parameter value M of strength-elongation balance, it is necessary that the ferrite fraction is more than 75%, the hard second phase is finely and uniformly dispersed and ferrite grain size is satisfactorily large.
When pearlite and bainite are mixed in the structure, the mechanical properties are considerably deteriorated.
In the practical hot-rolling operation, the actual necessary time from a final finishing roller to a coiler is about 10-40 seconds and the cooling means in a run-out table is limited to either of a laminate flow, water cooling through jet or air cooling, so that the hot-rolling process is less in the freedom for controlling the cooling condition than the reheating method and the hot-rolling method has a further problem in this point.
Therefore, in the case of the hot-rolling method, it is necessary to take into careful consideration so that the dual phase structure defined as described above can be obtained under the severely limited condition range.
Even though the actual transforming phenomenon caused in the hot-rolling step should be fully clarified and checked in order to overcome such a difficulty, it has never been attempted to fully check the three optimum conditions of the chemical components, rolling condition and cooling condition which are factors influencing upon the transforming behavior and further although these influencing factors have the mutual correlation, this point has never been taken into consideration.
Thus, when the prior hot-rolling method is checked in view of these points, said method has not been satisfied.
Discussion will be made hereinafter with respect to the problems of the already proposed main methods for producing the dual phase structured high tensile strength steel sheets through hot-rolling method and to the difference between these methods and the method of the present invention.
A first prior method, for example, Japanese Patent Laid Open Specification No. 34,659/80 or No. 62,121/80, provides that a part of the final rolling is carried out in a temperature range of two phases of .gamma.+.alpha. to effect a means for promoting the transformation of .gamma. into .alpha. owing to the strain induction and then a cooling condition in which stay time at a temperature range at which .gamma. is easily transformed into .alpha. is prolonged as far as possible, is adopted. However, in these methods, the drawback owing to the rolling in the two phase zone can not be avoided, so that when the rolling in the two phase zone is effected, ferrite phase and martensite phase in the final structure show the fiber-like dispersed state and anisotropy of mechanical properties due to this state is caused and the rolling strain remains in ferrite grains, so that the elongation property is deteriorated and the increase of the ferrite fraction mainly relies upon the increase of number of ferrite grains, so that the ferrite grains becomes fine and therefore the yield ratio becomes relatively higher.
In these methods, it has been difficult as mentioned hereinafter to obtain the steel sheets of a yield ratio YR.ltoreq.65% and a parameter M of strength-elongation balance=[0.45TS+El].gtoreq.60. A prior second method, as shown in, for example Japanese Patent Laid Open Specification No. 65,118/79 provides that after completing the final rolling at a temperature of higher than Ar.sub.3 point, cooling is discontinued when the temperature of a steel sheet becomes within a range of Ar.sub.3 -Ar.sub.1 in the course of rapid quenching of the steel sheet which has finished the final rolling at a temperature of higher than Ar.sub.3 point, on the run-out table, and the temperature is held for a given time and then the rapid quenching is again effected. This method intends to effectively progress the transformation of .gamma. into .alpha. during the intermediate holding time but does not cause the quality drawbacks as in the case of the above described rolling in the two phase zone and is an excellent idea in view of effective use of the limited time but even though the optimum cooling condition strongly relies upon the chemical components of the base material and the rolling hysteresis at the upper stream steps, these points are neglected and a mere two stage of cooling or a broad holding temperature range of Ar.sub.3 -A.sub.1 is only set, so that a high improvement of quality can not be attained. That is, the problem of the method of this prior art consists in that the countermeasurement regarding the above described points has not been yet clarified. When a trial calculation is made with respect to the examples in this publication, the quality level does not satisfy the yield ratio YR.ltoreq.60%, and the parameter M of strength-elongation balance=[0.45TS+El].gtoreq.60 and is substantially equal to that of the prior hot-rolling method.
In this prior publication, a simple C--Si--Mn system is only selected and it has never been noticed to use more advantageous C--Si--Mn--Cr system for forming the dual phase structure in view of the transforming property.